In a tunnelling machine having a rotary cutter head disposed on a front end of a cylindrical shield body, the rotary cutter head is provided with a face plate having one or a plurality of slits extending in the radial direction and a plurality of cutter bits disposed in the respective slits such that portions of the cutter bits extend from the slits. The rotary cutter head is rotated while the shield body is advanced to excavate natural ground with the cutter bits.
The length of each cutter bit projecting from the front face of the face plate is varied in accordance with the hardness of the natural ground to be excavated. When the natural ground is soft, it is preferable to make the projecting length of each cutter bit long to improve the operability of the tunnelling machine. On the other hand, when the natural ground is hard, the projected length is generally made shorter to ensure excavation by a predetermined energy input or applied force. Consequently, a tunnelling machine has been proposed in which the cutter bits are formed to be movable toward and away from the front face of the face plate (see Japanese Patent Publication (KOKOKU) No. 62-24597).
Such cutter bits respectively provided with forward bit portions and backward bit portions experience severe wear on their backward bit portions in the forward rotation and severe wear on their forward bit portions in the backward rotation. Accordingly, a rotary cutter head, having cutter bits mounted on the face plate swingably around an axis extending in the radial direction, has been proposed to reduce this wear (see Japanese Patent Publication (KOKOKU) No. 62-317).
Whenever the natural ground is composed of a very hard viscous soil layer or a mud stone layer, the natural ground may not be excavated by a forward and backward moving cutter head. The bit portions of cutter bits serving to excavate the natural ground, as well as bit portions of the cutter bits not serving to excavate the natural ground, should intrude into the natural ground while the cutter head is rotated in simple contact with the natural ground even if the cutter bits are pressed against the hard self-supporting natural ground as noted above and a turning force is given to the cutter head. Whenever the bit portions are forced to intrude into the natural ground, an extremely large thrust is needed, and the strength or hardness of each cutter bit should be increased.
When a rotary cutter head having swingable cutter bits is used, only bit portions serving for excavation are able to intrude into the natural ground.
However, since each cutter bit as described in the above-mentioned Japanese Patent Publication (KOKOKU) No. 62-317 is fixed to a bit seat swinging around a pivot pin supported on the face plate, the cutter bits are unable to move toward and away from the front face of the face plate. Thus, the length of each cutter bit projecting from the front face of the face plate is constant, and the tunnelling machine must in this condition cope with the excavation of either hard or soft natural ground. If the projecting length of each cutter bit is adjusted for excavation of hard natural ground, while attaching importance to safety, the efficiency during excavation of soft natural ground is decreased. On the other hand, if the projecting length of each cutter bit is adjusted for excavation of soft natural ground, while attaching importance to the efficiency of operation, the hard natural ground may not be excavated.
Furthermore, the force created during the excavation operation and exerted upon the cutter bits of the swingable type as noted above is received by the pivot pin, the bit seat, and the contact surface of the bit seat with the face plate. Thus, when particularly hard natural ground is excavated, a large bending moment is exerted to the pivot pin and may result in a defective swing of the cutter bits due to the bending of the pivot pin, thereby causing shear on the pivot pin.